Lecture 19 - Community Ecology- Competition

What determines where a species lives?

• dispersal
• abiotic conditions: climate and nutrients
• species interactions: competition, predation, mutualism,

What are the different types of species interactions and their oversimplification?

• competition: bad for both species A and B
• predation: good for species A and bad for species B
• mutualism: good for both species A and B

What do ecologists and evolutionary biologists study?

ecologists - population dynamos and effects on community
- how species interact

evolutionary biologists - evolutionary dynamics
- adaptation, coevolution
how species interactions affect adaptations

What is INTRAspeific competition? What does conspecifics mean?

• competition among the same species for resources
• individuals of the same species

What is INTERspecfic competition? What does heterospecifics mean?

• competition among different species for resources

• individuals of different species

What is scramble/exploitative competition?

-depletion of shared resource

• individual consuming a resources, leading less for others

What is contest/interference competition?

direct interactions, battles over territory

What is an example of interference/contest competition?

invasive Argentine ants are superior competitors

• drive down populations of native species
• compete for space

What is an example of exploitative/scramble competition?

• two species don't interact directly
• one consumes resource, leaving less for another species

How can we model INTER specific competition?

• Lokta-Volterra equations for two species competing for resources

What is the Lotka-Volterra model for interspecific competition? How do we model it?

1. start with logistic model for population growth (intra)

dN/dr = rN(1-N/K)

2. rewrite the logistic model with subscripts to indicate species

dN1/dt = r1N1 (1-N1/K1)

3. add terms to show affect of species 2 on 1

dN1/dt = r1N1 (1-N1/K1 - a12N2/K1)

• models effect of completion one species has on species 1
• as more species 2, its going to slow population growth for species 1

What is the final Lotka-Volterra Model Equation? What do they mean?

dN1/dt = r1N1 (1-N1/K1 - a12N2/K1)

• a12 represents the per capita effect that species 2 has on species 1

dN2/dt = r2N2 (1-N2/K2 - a21N1/K2)

• a21 represents the per capita effect that species 1 has on species 2

What does the alpha term mean?

per capita effect on a on b

• competition coefficient

What does a12N2 do? Give an example?

converts individuals of species 2 into equivalent number of individuals of species 1

• squirrel eats more seeds than sparrow

• a is how many sparrows amounts of seeds a squirrel can eat

• sparrow eats less than squirrel

• a is how many squirrels worth of seeds a sparrow can eat

What are possible outcomes of LV competition? What does it depend on?

• species are able to coexist
• species 1 always wins (N1=K1, N2 = 0)
• species 2 always wins (N1=0), N2 = K2)
• depends on initial populations

What does coexistence require?

• intra competition be more stronger than inter competition
• outcomes depend on K value and a values

What does equilibrium mean?

population is not changing over time (dN/dt =0)

• community is at equilibrium if the same species are present as a year ago

What does stability mean?

• ability of system to return back to equilibrium after a disturbance
  ex. hurricanes, wildfire

What does coexistence mean?

• occurs when two or more species have non-zero population sizes at equilibrium
• they persist in a community

What are principles of competitive exclusion?

• for two species to coexist, competition btw them must be weaker than competition within species

• two species can't compete too much or one will overpower the other

• complete competitors cannot exist

What leads to competitive exclusion?

limited resources, food, or space generate competition among individuals

What is niche partitioning? Hint: warblers (birds)

• exist by partitioning resources
  • feed in different parts of this trees

What did Robert MacArthur experiment?

• measured how much time each species spent in different parts of the trees
• feed in different parts
• evidence that similar species coexist in communities but in diff niches
• don't violate the principle of competitive exclusion

What is character displacement? Hint: finches. What effect does this have on completion between species?

• trait evolution

• coexisting pieces evolve differences

• living separately they have overlapping beak sizes

• islands that have both, don't have overlapping beak sizes

• one has evolved smaller, one evolved larger

  • reduces competition

• how competition selects organismal traits

• how competition for resources led to this trait adaption

What is the paradox of the plankton?

• principle of competitive exclusion is false for plankton
• plankton coexist but also compete fore the same resources
• phytoplankton need light, CO2, N, P, and micronutrients

What is the paradox of tropical rainforests?

• each species has distinct Niche
• something prevents competitive exclusion from driving species extinct

How do LV models relate to the real world?

• studied competition among protozoa
• saw stable coexistence and competitive exclusion

What was Gauses's experiment? What do these graphs show us?

• grew three different bacteria species
• placed first two tgt and saw competitive exclusion
• place other two together and saw stable coexistence

How are competitive effects manifested in nature?

• competitive exclusion is less likely to go to completion
• different outcomes in different environments

What is an example of testing competition experimentally? Hint: barnacles. What was the result of the experiment?

• upper zone had Chtamalus to thrive because of desiccation
  • balanus can't toleration being out of water, risking desiccation
• lower zone had balanus by competing for space

What does competiton effect?

• affects which and how may species occur in an ecological community

• community composition : which

• species richness : how many

What is true about LV models and real communities?

• LV models are too simple
• communities are not at competitive equilibrium

How are real populations kept below carrying capacity?

because of weather, diease and predators

• conditions fluctuate and favour different species at diff times